Long-term recovery of soft-bottom benthos following urban and industrial sewage treatment in the Nervión estuary (southern Bay of Biscay)

Ángel Borja*, Iñigo Muxika, Javier Franco

ABSTRACT: The monitoring of marine benthic communities is based upon their ability to integrate long-term environmental conditions and it is considered an efficient, accurate and useful tool in the detection of pollution effects. However, long-term studies are scarce, and only few are devoted to recovery processes in estuaries following water treatment. In this contribution, the response of the Nervión estuary to sewerage processes is studied, using a long-term (1989 to 2003) data set of physico-chemical (dissolved oxygen, redox potential, organic matter, metals) and biological (density, richness and diversity in benthic communities) variables. The AMBI (AZTIs Marine Biotic Index) was designed to investigate the response of soft-bottom communities to natural and human-induced changes in European coastal water quality and is evaluated here. The first factor controlling the Nervións benthic structure is found to be the bottom water oxygen saturation, hypoxia- and/or anoxia-produced azoic sediments. When water treatment commenced, benthic recolonisation was very rapid in terms of opportunistic species settlement. However, the community structure was highly disturbed, in terms of density, richness, diversity and AMBI values. AMBI and diversity values are highly correlated with oxygen saturation. When the water treatment continued, the high metal concentrations in the sediments inhibited recovery in some areas; they were a secondary factor in determining benthic structure. This contribution demonstrates that the AMBI is an efficient tool for detecting changes in benthic communities and in systems associated with sewerage schemes; it provides an indication of the proportions of opportunistic and sensitive species within a long-term series.